Polypropylene exhibits several desirable properties, for example, relatively low density excellent resistance to deformation at higher temperatures and resistance to aqueous and nonaqueous liquids. Polypropylene also has a less favorable characteristics such as inadequate impact strength at temperatures below room temperature especially below 0.degree. C. Adequate impact strength, however, is required and is of importance in many uses such as, (for example), freight containers, suitcases, automobiles parts and similar applications. Polyethylene of high density of which such molded articles are often made, possesses satisfactorily high impact strength at low temperatures, but a lower resistance to deformations at high temperatures. Ethylene propylene copolymers, saturated as well as unsaturated, exhibit good mechanical properties, high ageing resistance, high ozone resistance as well as impact resistance at low temperatures, such that the copolymers are also excellently suited for use where the product is exposed to weathering. Due to the elastomeric properties of these copolymers, it is popular to combine such copolymers with harder polymers. Many attempts have been made to combine the properties of polypropylene and/or polyethylene and/or ethylene propylene copolymers by the production of binary or ternary blends or molding compositions. Mixtures of polypropylene and polyethylene are known, however, these mixtures exhibit an impact strength which is hardly improved over polypropylene itself. Also, blends of isotactic polypropylene and ethylene propylene elastomers, which are cross-linked or partially cross-linked by vulcanizing agents, require improvement with respect to tensile strength, tear propagation, and hardness values attainable.
Blends of polyethylene with EPDM terpolymers of ethylene, propylene and nonconjugated diene are known from U.S. Pat. No. 3,919,358 and exhibit high tear strengths. The blends do not show sufficient heat resistance due to the low melting temperature polyethylene. Ternary molding compositions are also described in British Pat. No. 1,154,447. These crystalline polypropylene, polyethylene and ethylene propylene block copolymer ternary blends exhibit a tensile strength of only 10 N/mm.sup.2 and are not flexible at temperatures below 30.degree. C.
Various polyolefin ternary blends have still been used in various industrial applications because of the balance achieved among several properties, for example, between rigidity and impact resistance. In particular, ternary blends known as thermoplastic elastomers (TPE's) are used where rigidity and impact resistance are important. Typically a thermoplastic elastomer is a blended product of a hard segment of crystalline polypropylene or crystalline polyethylene and a soft segment of an olefin elastomer such as ethylene propylene rubber (EPR), ethylene/propylene/diene terpolymer (EPDM), polyisobutylene or polybutadiene. The TPE may also be subject to partial cross-linking in order to improve physical properties. In recent years however, the required merits of the blend such as good moldability, appearance of the molded articles i.e. high gloss, paintability, thermal resistance, low temperature impact resistance and the property that the deformation caused by impact is easily restored have become more important. The conventional TPE's have not always met these requirements.
U.S. Pat. No. 4,748,206 to Nogiwa discloses quaternary blends of polypropylene, ethylene propylene rubber, Ultra Low Density Polyethylene (ULDPE) and Linear Low Density Polyethylene (LLDPE) or Medium Density Polyethylene (MDPE) and addresses some of these issues. However, the inclusion of 5 to 30 wt. % MDPE and 5 to 50 wt. % ULDPE is limiting. Spielau et al. in U.S. Pat. Nos. 4,319,004 and 4,319,005 disclose ternary blends of 38 to 48 wt. % non-amorphous ethylene propylene copolymer (at least 65 wt. % ethylene), polyethylene, preferably High Density Polyethylene (HDPE) and 3 to 30 wt. % non-amorphous propylene homopolymer or (polypropylene copolymer up to 12 wt. % ethylene).
Further Kobayashi et al (U.S. Pat. No. 4,822,855) disclose a blend of 30 to 70 parts polyethylene, 30 to 70 parts of random propylene copolymer of up to 12 wt. % ethylene and 70 to 200 parts ethylene propylene rubber. Kobayashi, however, uses very low density polyethylene (VLDPE) with a density between 0.86 to 0.91 g/cm.sup.3.
U.S. Pat. No. 4,990,554 discloses a blend of (a) 75 to 97 wt % of a blend of (1) 90-65 parts crystalline polypropylene (2) an ethylene copolymer (7 or less percent by weight ethylene) and/or propylene homopolymer and (3) 10-35 parts polyethylene and (b) 25-3 wt % filler.
U.S. Pat. No. 4,948,840 discloses a blend of (1) 10-70 parts by weight of propylene (2) 20-60 parts of EPR (3) 10-30 parts semi crystalline, low density, essentially linear ethylene propylene copolymer and (4) 3-15 parts 1,2 polybutadiene and a peroxide crosslinking agent.
U.S. Pat. No. 4,88,775 discloses 50-96 wt % impact modified polypropylene polymer, 2-45 wt % of HDPE homopolymer, 2-45 wt % LLDPE and 0-30 wt % EPR or EPDM.
There still exist a need in the art to develop a rubber toughened thermoplastic olefin for use in molded applications that has a higher MFR which leads to easier moldability.
Indeed, many compositions with ethylene/propylene rubber in them would have very good physical properties if they could be molded. However, the increase in ethylene/propylene rubber proportion in these blends tends to increase viscosity, i.e., decrease the overall melt flow ratio of the mixture. Thus, a high amount of ethylene propylene rubber makes a TPE blend difficult if not impossible to process. Thus, there still exists a need in the art to develop a rubber toughened thermoplastic olefin for use in molded applications that can be more easily molded.